The determination of the state characteristics of oils (including fats but not limited to use for food preparation) is in demand in many fields. In the engine, hydraulics, transmission and turbine fields, especially in the area of motor vehicles, for example, a great need exists to measure in particular the oil quality as well as the oil temperature of the oil in circulation. Oil can, for example, oxidize in the engine of a motor vehicle due to the air penetrating the crankcase, so that in the process acids can form. Separated oleoresins and asphalts as well as street dust, metallic abraded particles and dissolved combustion residues sludge up the oil. The circulation of oil can be additionally impeded by condensation water and sometimes coolant. As a result the oil quality and oil degradation make it possible to determine whether an oil change is necessary to conserve the machine components.
A known oil sensor of TEMIC company is able to measure the dielectric constant, the level as well as the temperature of the oil in an oil pan of a motor vehicle. For this purpose the sensor cell exhibits two cylindrical capacitors, of which one is completely immersed in the oil. Its capacity depends on the dielectric constant of the oil. The second capacitor is located in a position at which the oil state can be determined between the expected maximum and minimum level. The capacity of the second capacitor depends on the dielectric constant of the oil and the level in the oil pan. The sensor cell can be mounted in the oil pan with the help of a mechanical adapter. The sensor cell is connected via electronic cable to measuring electronics outside of the oil pan. The sensor cell is for this purpose guided through the oil fillers and screwed to the filler with the help of the mechanical adapter.
A disadvantage of the known oil sensor is the fact that its operation is relatively awkward. The sensor must be removed prior to the refilling of the oil. A further disadvantage consists in the fact that the oil in the oil pan does not necessarily represent the oil that is in circulation and therefore does not furnish a clear picture of the actual lubricant. The metal of the oil pan as well as that of the crankshaft can constitute an additional problem which can lead to inductive currents that influence the measuring accuracy.
In addition, from DE 101 03 532 A1 as well as from DE 100 25 690 A1 it is known to arrange a sensor close to the inside wall of a filter housing of an oil cycle. In the case of these known arrangements, among other things, the accessibility to the sensor is restricted or the expenditure for replacing the sensor is quite high. Similarly, it holds true also for the filtering apparatus disclosed in US 2003/0046985 A1, which discloses a cylindrical linear arrangement of several electrodes at the inside wall of a filter housing. Here too the accessibility and handling of the measuring device is not easy. In addition the measuring accuracy of these oil sensors is not optimum.
DE 102 08 600 A1 describes a sensor integrated in an oil filter which can only be replaced in conjunction with replacement of the oil filter. This device is correspondingly expensive and not to be favored in particular due to cost aspects.
U.S. Pat. No. 5,023,133 discloses an oil filter with a hollow cylindrical filter element into whose interior an acid sensor is placed. From the rim of the filter housing wires are conducted to an ohmmeter. This sensor is neither easily accessible nor easily replaceable.
In addition, precise measurement of the oil state characteristics is not possible with any of the known measuring devices.
In other fields, in particular in the field of food, the measurement of state characteristics of fats is desirable or even prescribed by law. Thus there is a great need for example in the case of frying fats for a simple and precise means of determining their quality as well as their temperature or other measurable fat characteristics.